Adult Ascaris body muscle succinate dehydrogenase acts as a fumarate reductase and the reduction of fumarate is coupled with oxidation of NADH and phosphorylation of ATP in this anaerobic functioning tissue. In contrast, succinate dehydrogenase in the 1-celled fertilized egg, developing embryo of Ascaris and its host functions in opposite but conventional directions in the tricarboxylic acid cycle and is exclusively associated with electron transport and oxidative phosphorylation of the inner mitochondrial membrane. Because of this profound difference in this key metabolic enzyme of host and parasite, a detailed study of succinate dehydrogenase from the 1-celled fertilized egg, 10-day embryo, adult body muscle of Ascaris and rat liver is proposed. Mitochondria from both parasite and host will be isolated by rate zonal centrifugation, SDH solubilized by treatment with deoxycholate in the presence of a chaotrope (NaClO4), partially purified by precipitation with (NH42), SO4 and further purified by DEAE-cellulose column chromatography. Differences between parasite and host SDH will be noted. Such differences may afford opportunities to design chemical agents that interfere with the function integrity of parasite without injury to the host for safer treatment of this disease. Therefore, information stemming from the proposed research could possibly be used in a positive fashion for chemotherapy of other parasitic diseases.